1. Field of the Invention
The present invention relates to an optical add/drop multiplexing technique and its control program, and to a technique effectively applicable to an optical add/drop multiplexer and its control technique constituting an optical network that utilizes a wavelength division multiplexing (WDM) for example.
2. Description of the Related Art
In recent years, in proportion to the expansion of telecommunications capacity and range, the large capacity and long distance capabilities of a network have become increasingly in demand. An optical network utilizing a wavelength division multiplexing (WDM) has conventionally been used as a backbone network in order to satisfy such demands.
An optical network utilizing the WDM uses an optical add/drop multiplexer (OADM) for adding a discretionary wavelength to a discretionary path, and dropping and receiving a signal light of the discretionary wavelength from the discretionary path in order to meet a requirement of telecommunication between cities.
The WDM usually uses an optical amplifier such as an EDFA (Erbium-Doped Fiber Amplifier), et cetera, for amplifying a wavelength range including wavelengths of a plurality of signal lights. Such an optical amplification generates an ASE (amplified spontaneous emission) noise and therefore a countermeasure against a failure caused by an accumulation of ASE noise is necessary in the case of using a plurality of OADMs by connecting them in multiple stages.
For instance, a patent document 1 has disclosed a technique for attempting to remove an ASE noise, excluding signal lights, by placing an optical filter synchronously with each signal light in an optical path for each signal light between an optical drop filter and an optical multiplexer in an OADM.
And the patent document 1 has also disclosed a technique for attempting to maximize each signal light level by placing an optical monitor for detecting an optical power of each signal light in the optical path of an optical multiplexer on its output side and feeding back information of the observed optical power to an optical filter control unit.
A patent document 2 has disclosed a technique for attempting to prevent a cumulative increase of an ASE noise by using different free spectral ranges between an optical branching filter and an optical multiplexer in an OADM.
However, neither of the above noted patent documents 1 nor 2 recognizes the technical problems at the time of adding a signal light as follows.
That is, the WDM requires a control of an intensity of each signal light of a plurality of optical wavelength signals which are set at a predetermined frequency interval so as to identify with a target level and a control for adjusting an inserted signal light to the target level in the case of adding a new signal light.
Accordingly a conceived method is to make each OADM carry out the control for maintaining a standby state until the inserted light reaches a first threshold level value, transitioning to an automatic control state of a signal level when exceeding the first threshold level value and transitioning to a standby state if the signal level becomes lower than a second threshold level value that is higher than the first threshold level value in the automatic control state.
A normal control of a target level in the automatic control state is carried out by adjusting an attenuation degree of an optical attenuator installed in the optical path of each signal light.
The ASE noise, however, is made to shift so as to increase an overall signal level across almost the whole range of a wavelength multiplexed light, and therefore the ASE noise may be accumulated while passing-through a plurality of OADMs, which are connected in multiple stages, and may exceed the above described first threshold level value since a signal level in an unused wavelength band as the subject of insertion may have increased.
Consequently, if a transition between the above noted standby and automatic control states is controlled by a detection of a signal level in a plurality of OADMs which are connected in multiple stages, an OADM which is positioned as a subsequent stage of an OADM that adds a signal light and makes the signal light pass through assumes a state of transitioning to an automatic control state if a level of a signal light of the incoming inserted wavelength band exceeds the first threshold level value due to an ASE noise.
And, since the ASE noise level in this event is lower than the target level, the control system in the automatic control state assumes a state of waiting with the degree of attenuation of the attenuator at the minimum in order to make the ASE noise level reach the target level.
Then, when carrying out a signal light insertion, by connecting an optical fiber of an OADM of the previous stage, to the OADM which is in the automatic control state, there is a technical problem of a power level of the inserted signal light in the OADM on a pass-through side exceeding the target level, because the degree of attenuation is at the minimum, thus bringing about a data error such as crosstalk by disturbing a signal level of other signal lights of the adjacent wavelengths.
In order to avoid such a technical problem, a possible approach is to avoid an accumulation of the ASE noise by limiting the number of connecting stages of the OADM this results, however, in the technical problem of limiting a telecommunication range of a telecommunication system using the OADMs.
[Patent document 1] Laid-open Japanese Patent Application Publication No. 2002-204209
[Patent document 2] Laid-open Japanese Patent Application Publication No. 2003-69496